Google Domains because I have a Google account and buying a domain on it was easy when I needed it. I'm still on Google Domains but you've reminded me I need to continue the transfer to Cloudflare before I get forced over to Square Space because they don't support Dynamic DNS.
Transferring was straightforward enough, but there were a couple steps that involved waiting for things to update before you could continue and I forgot to get back to it for a while after they were done. Other than that, all my records seem to have transferred over correctly and all I had to do manually was reconfigure my DDNS client and set up email forwarding with gmail again.